1. Field of the Invention
The present invention relates to a transmitter. More specifically, the present invention relates to a transmitter for bi-directional microwave communication from general households or small-scale offices to broadcast satellites or communication satellites.
2. Description of the Background Art
The market for radio communication utilizing microwaves has been recently developed dramatically, along with developments of various systems including broadcast satellites and communication satellites. At the same time, the Internet has been developed and digital satellite broadcast has started, ever increasing the demand for bi-directional communication.
For bi-directional communication between a small-scale office or a general home and a broadcasting station, a broadcast satellite or a communication satellite is used. Currently, it is a dominant practice to use the satellite broadcast, as a signal transmission path for a down signal (downstream) from the broadcasting station to a general home, and to use a telephone line, as a signal transmission path for an up signal (upstream) from a general home to the broadcasting station.
The telephone line used for the upstream supports only a slow rate of communication, and therefore it is not suitable for exchanging motion picture, for example, hindering promotion of satellite multimedia applications. Thus, there has been an increasing necessity to introduce satellite communication not only to the downstream but also to the upstream transmission.
FIG. 6 shows a concept of bi-directional communication between each home and the broadcasting station through satellite broadcast.
Referring to FIG. 6, a parabola antenna 51 is provided on the roof, for example, of a broadcasting station 50, and parabola antennas 62 and 63 are provided on the roofs of homes 60 and 61, respectively. Through broadcasting satellite 70, microwave bi-directional communication is performed between each of the parabola antennas 62 and 63 of respective homes and parabola antenna 51 of broadcasting station 50. For bi-directional communication, microwave of 12 GHz band is used for one direction and microwave of 14 GHz band is used for the other direction. An LNB (Low Noise Block down Converter) similar to the one used in the conventional system for satellite broadcast reception is used as a receiver for bi-directional communication, and a transmitter is newly provided.
FIG. 7 is a block diagram of such a transmitter. Referring to FIG. 7, an indoor unit 20 converts a signal from a terminal such as a personal computer to a video signal, and outputs the same as an intermediate frequency signal of the band between 950 MHz to 1450 MHz to a coaxial cable 21. A DC voltage of 13V to 26V, for example, is supplied to coaxial cable 21, and the intermediate frequency signal is superposed on the DC voltage and supplied to a transmitter 30.
Transmitter 30 is positioned close to a parabola antenna provided on a roof, for example. The DC voltage supplied to coaxial cable 21 is converted to a prescribed voltage by a DC-DC converter, and supplied to various circuits.
The intermediate frequency signal is amplified by buffer circuits 31 and 32 to ensure a gain, and supplied to a mixer 33. To the mixer 33, a local oscillation signal is applied from a local oscillation circuit 34 through an amplifier 35, and by the mixer 33, the local oscillation signal and the intermediate frequency signal are mixed and converted to a microwave signal of 14 to 14.5 GHz. The microwave signal has its spurious component removed as it passes through a bandpass filter (BPF) 36, amplifiers 37, 38 and 39 formed of HEMTs (High Electron Mobility Transistors), a bandpass filter 40 and an amplifier 41.
Further, the microwave signal is amplified by a driver amplifier 42 to ensure a gain, has its components other than the high frequency signal removed by a highpass filter (HPF) 43, amplified by a high power amplifier 44 and supplied to an isolator 46 through a highpass filter 45. Thereafter, the microwave signal is transmitted to the broadcasting satellite from isolator 46 through the parabola antenna.
Once transmitter 30 shown in FIG. 7 is installed in a general household, bi-directional communication through the broadcasting satellite becomes possible. However, indoor unit 20 and transmitter 30 are expensive, and hence use thereof has not yet been widely spread among ordinary households. One reason why indoor unit 20 and transmitter 30 become expensive is that it is necessary to adjust power level of transmitter 30 with the level of the intermediate frequency signal output from indoor unit 20.
As transmitter 30 has only the function of converting the intermediate frequency signal to a microwave and amplifying the same, the level of the intermediate frequency signal input from indoor unit 20 to transmitter 30 varies dependent on the environment of installation, such as the length of the coaxial cable 21 between indoor unit 20 and transmitter 30. Therefore, there must be some means to adjust the output level of intermediate frequency signal output from indoor unit 20, which leads to higher cost.